Digital vs Analog Volume Control

With the rise of computer-based high-end audio playback, a very interesting question is whether is is best to perform volume control in the digital or analog domain. I have done some experiments in this area, and I thought I might share some of my thoughts on the subject.

The very best DAC/preamplifier combos have sufficiently low noise that they can resolve perhaps the 21st bit and even the 22nd bit of an audio signal. You would have to choose between that and buying a fast car to achieve that level of performance! The merely “very good” are capable of resolving the 19th to 20th bit as a rough guideline. If you implement volume control in the digital domain, every 6dB of attenuation results in the loss of one bit of resolution. If you play back music music with 24-bit bit depth, then all volume control results in irrecoverable loss of data via bit-depth reduction. However, if you play 16-bit music, and pass it to your DAC in 24-bit format (something which most high-end DACs require anyway), then, depending on the quality of your DAC/preamp combo, you can in principle dial in up to 18-36dB of attenuation without audibly truncating the the music data. All this assuming that your digital volume control is done in a first-class manner using a 64-bit audio engine or its equivalent, as implemented in BtPerfect..

So that’s the theory.

On the other hand, volume control performed in the analog domain requires passing the signal through some sort of variable attenuator – either a potentiometer or an active electronic equivalent, or a switched resistor ladder. These components do actually degrade the sound, and to quite an alarming degree! If you are in the habit of “tweaking” your audio equipment, you will know that a hardy market exists for after-market volume control potentiometers costing up to thousands of dollars each (!!!) to try and eliminate these sonic defects. So the answer to the question boils down to whether or not the sonic degradation introduced by Bit Depth reduction is less intrusive than that introduced by a preamplifier’s volume control.

As it happens, I have done some extensive listening tests on this subject, and I have surprised myself by the conclusions I have drawn. Regardless of whether the music is 16-bit or 24-bit, I have found that performing volume control in the digital domain is qualitatively superior to performing it in the analog domain. And the difference is not subtle – it is really quite massive. No contest, actually. I will temper that statement by saying that it for sure depends on the preamplifier you are using and the volume control circuitry it implements. For example, I had a chance to discuss this with Dan d’Agostino, and while he agrees with me, he assures me that his new $30,000 preamplifier has a volume control that introduces no sonic degradation whatsoever!

There is a significant practical downside to performing volume control in the digital domain. Basically, your DAC is connected to a preamplifier permanently set to maximum volume. Depending on the rest of your audio equipment, the consequences of accidentally playing music at maximum volume may represent a risk that you are just not willing to take. Most computer playback systems have a user interface that has not been designed with this concern in mind! You would have to be very particular indeed about the procedures you go through each time you start to play music.

In my case, I run my Light Harmonic Da Vinci DAC directly into the inputs of a 300W/ch Classe CA-2300 power amplifier, feeding B&W 802 Diamond loudspeakers. All of my serious listening is done with about 20-30dB of attenuation dialled in by BitPerfect using the iTunes volume slider. I find this to be massively superior to routing the signal through my Classe CA-800 preamplifier with all digital attenuation turned off and a truly “bit perfect” signal passed into, and attenuated by, the preamplifier.

As this subject is taken up by a wider audience, and more different system configurations are evaluated, it will be very interesting to see what sort of a consensus emerges.